In an exclusive interview with The Sun (subscription required—quotes here via The Guardian), Swann explained his decision:

I came to Australia in the hope of winning our fourth Ashes series on the trot, but that's now impossible. So it's time for someone else to buckle in and enjoy the ride.

I could have made myself available for the last two Tests and maybe had a bit of a send-off from the Barmy Army. But that's no reason to hang on for two more games. I like to be the centre of attention—but for the right reason: by winning games of cricket.

Swann has struggled with form this series and over recent years has struggled with elbow injuries which have required surgery.

Former England captain Michael Vaughan was one of the first to pay tribute to Swann's career:

Too soon IMO for Graeme Swann 2 retire but it's been a absolute pleasure watching him perform for many years.Finger spin out of the top draw

Swann was a late bloomer at international level. He first toured with England in 1999-2000, but as Simon Briggs of the Daily Telegraphrecorded, he did not fit well with then coach Duncan Fletcher and had to wait until 2008 for an opportunity at Test level.

From the moment he made his debut, however, he was an integral part of the team:

Graeme Swann played his first Test in December 2008, since which England have played 66 Tests, he has played 60 of them. #Ashes

Swann's Test record is, for a finger-spinner, exceptional. He is sixth on the all-time list of England wicket-takers, with only Derek Underwood boasting more scalps as a spinner.

Graeme Swann: International Bowling Record

Mat

Inns

Balls

Runs

Wkts

BBI

BBM

Ave

Econ

SR

Tests

60

109

15349

7642

255

6/65

10/132

29.96

2.98

60.1

ODIs

79

76

3809

2888

104

5/28

5/28

27.76

4.54

36.6

T20Is

39

38

810

859

51

3/13

3/13

16.84

6.36

15.8

Cricinfo

In the short term, England can cover his absence by playing Monty Panesar at Melbourne on Boxing Day. Alternatively England could field an all-seam attack, with a trio of tall pace-men—Boyd Rankin, Steven Finn and Chris Tremlett—all with the touring squad.